1. Field of the Invention
This invention relates to signal generators. More specifically, the present invention relates to oscillator circuits employing switchable current sources which provide substantial improvement over linear feedback oscillators.
While the present invention is described herein with reference to a particular embodiment for a particular application, it is understood that the invention is not limited thereto. Those of ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications and embodiments within the scope thereof.
2. Description of the Related Art
RLC (resistive-inductive-capacitive) tank circuits have long been used as frequency determining elements for many applications. In some applications, such as metal detection, there has been a need for a signal source which, when coupled with a tank circuit, provides a highly stable oscillation with a fast start-up time. In metal detection, for example, the movement of metal in the vicinity of the tank circuit causes a change in the inductance of the inductor and a corresponding change in the resonant oscillating frequency of the tank circuit.
Linear feedback oscillators have been used with limited success in this capacity. A linear feedback oscillator is essentially an inductive-capacitive (LC) tank circuit in parallel with a voltage controlled current source. A resistive element is provided by the internal resistance of the inductor and the current source. An amplitude detector monitors the amplitude of the output voltage of the tank circuit and feeds a signal back to a low pass or bandpass filter. The output of the low pass or bandpass filter controls the gain of the voltage-controlled current source.
Whatever the implementation, two important characteristics of such an oscillator are its start-up mechanism and its equilibrium condition. These factors are interrelated inasmuch as the transconductance of the current source must equal the reciprocal of the equivalent parallel resistance (R) of the tank circuit, current source combination to maintain a state of equilibrium. However, conventional signal sources are such that the start-up of the oscillator is a consequence of the presence of a random noise voltage. If the transconductance g.sub.m &gt;1/R, the amplitude of the oscillation grows exponentially up to the limit of the power handling capacity of the electronic devices. Thus, for optimum performance, conventional linear feedback oscillators typically have a high g.sub.m to start and a lower g.sub.m (i.e., equal to 1/R) to maintain equilibrium.
Unfortunately, since the random noise voltages are extremely small, the time required to reach a nominal amplitude level is usually considerable. More importantly, this time period is typically not constant and it is therefore very difficult to compensate for its variation. While the necessary time period may be reduced by providing a high g.sub.m at start-up, there is no provision in the art for overcoming the basic uncertainty in the start-up time. In addition, there are constraints on the extent to which the transconductance may be increased for start-up. Also, if the transconductance is set too high for the equilibrium condition, then amplitude clipping of the resultant output due to power supply limitations may result which may cause distortion and undesirable shifts in the frequency of oscillation.
Accordingly, there is an need in the art for a signal source which, when coupled with a tank circuit, provides a well-controlled oscillator amplitude having a predictable start-up time.